Increasing efforts have been made to purify the exhaust gases which become available at high rates, e.g. in refuse incinerator plants or power plant boilers, and to avoid air pollution by smaller exhaust gas sources.
In steelmaking plants, such smaller exhaust gas sources can include tipping troughs, transfer pits, casting ladles and the like as well as various grinding units and may produce exhaust gases at a rate of up to 150 m.sup.3 /sec. Even though these gases often become available only periodically, they must be cleaned before their discharge into the atmosphere because they would otherwise add substantially to the pollution of the environment.
In the design of electrostatic precipitators for such applications it must be borne in mind that dust is to be collected from gases which have a very high dust content with a large proportion of fines and that even this very fine dust must be collected if the gas is to be cleaned to the high degree which is desired and is required by law.
It is known that very fine dusts cannot be collected in electrostatic precipitators unless the latter comprise a plurality of separating zones successively traversed by the gas and in which discharge conditions can be selected to match the different dust resistivities. Thus the several high-voltage zones must be separately controlled.
A multizone design is also desirable to avoid a failure of the entire plant when a single corona-discharge wire has broken. By law, in some jurisdictions, the plant must be so designed that all dust in excess of, e.g. 150 mg/m.sup.3 is removed from the gas even when one zone has failed.
It is also desirable to minimize the capital investment. For this purpose, most electrostatic precipitators used for such purposes are arranged for a vertically rising gas flow so that no exhaust gas blowers are required in most cases because the natural draft of the hot gases is sufficient to overcome the pressure drop in the electrostatic precipitator.
However, electrostatic precipitators traversed by the gas vertically have the basic disadvantage that special structure is required to remove the collected dust from the collecting electrodes so that the collected dust is not raised and entrained by the following gas. In most cases, double-wall collecting electrode panels are used, these panels being provided with horizontally extending dust-receiving means, such as slots, pockets or the like, which deliver the dust to the space between the walls of the double-wall electrodes. From that space, which is not traversed by the gas, the dust is discharged by gravity out of the electrostatic precipitator.
The added cost of such collecting electrodes more or less offsets the decrease in capital investment resulting from the elimination of the need for a blower or from the use of only a very small blower for assisting the natural draft or the plant is even more expensive.
Reference may also be made to U.S. Pat. Nos. 3,803,809, U.S. Pat. 4,058,377 and U.S. Pat. 4,035,886, which relate to strip-type collector electrodes and corona-discharge electrode support frames (see also U.S. Pat. No. 3,745,620).
These patents and the literature or references cited in the files thereof demonstrate that the formation of electrode walls and corona-discharge electrode systems are known in the art.
Nevertheless, there has not been provided heretofore, to my knowledge, any vertical flow electrostatic precipitator which is of the desired high efficiency, low cost and simplicity long sought after in dust-cleaning applications in which a large proportion of fines is included in the dust to be removed from the gas.